1. Field of the Invention
This invention relates to a shim support guide jig for a magnetic field generation apparatus, a magnetic field generation apparatus and a magnetic resonance imaging equipment each including a shim support in which magnetic material shims are arranged and adjusted by employing a shim support guide jig, and a magnetic field adjustment method for a magnetic field generation apparatus, as well as a magnetic field adjustment method for a magnetic resonance imaging equipment.
2. Description of the Related Art
A magnetic field generation apparatus including a superconducting magnet is used in a magnetic resonance imaging equipment. The superconducting magnet is configured in a cylindrical shape, and it generates a static magnetic field in its internal space. A patient is arranged in the static magnetic field, and the image diagnosis of the patient is made.
A magnetic field whose magnetic field strength is high, which has a uniform magnetic field strength in the internal space, and which is of high temporal stability, is required of the static magnetic field. The superconducting magnet is used for generating such a static magnetic field. Concretely, it is required of the superconducting magnet for the magnetic resonance imaging equipment to generate a magnetic field which has a very uniform magnetic field strength of, for example, within 3 ppm, in a magnetic field space near the center of the magnetic field. Therefore, the superconducting magnet is designed at a high precision in correspondence with the requirement. In actuality, however, the uniformity of the magnetic field in the magnetic field space lowers due to a manufacturing dimensional error in a process for producing the superconducting magnet, the influence of a magnetic material existing around a place where the superconducting magnet is installed, and so forth.
For this reason, the superconducting magnet is additionally provided with a magnetic field adjustment device for making the fine adjustment of the magnetic field strength. The magnetic field adjustment device employs magnetic material shims made of a magnetic material of, for example, high permeability. The magnetic material shims serve to correct the disorder of the magnetic field strength of the static magnetic field generated by the superconducting magnet, and they are arranged and adjusted on the basis of the distribution evaluation of the magnetic field strength. More specifically, the magnetic material shims have a designative volume which is designated on the basis of the distribution evaluation of the magnetic field strength, and they are arranged at a designative position which is designated on the basis of the distribution evaluation. The disorder of the static magnetic field generated by the superconducting magnet is corrected with a magnetic field which is generated by the magnetization of the magnetic material shims, whereby a highly uniform magnetic field is obtained. Regarding the magnetic material shims, Patent Documents 1 (JP63-122441A, especially FIGS. 1 and 2 and their explanations), 2 (JP5-220127A, especially FIG. 2 and its explanation), and 3 (JP5-329128A) have been known.
FIGS. 1 and 2 of Patent Document 1 disclose a magnetic field strength correction device wherein a pair of protrusions are formed through leaf springs on both the sides of a holder for holding the magnetic material shims, and the holder is arranged at the predetermined position of a holding member by using the protrusions. Besides, FIG. 2 of Patent Document 2 discloses a magnet device wherein a slide plate is disposed between a pair of groove-shaped members, and the magnetic material shims are arranged on the slide plate, whereby the mounting position of the magnetic material shims can be freely set. Further, Patent Document 3 discloses a magnetic field correction device wherein a plurality of flat magnetic material members which are equal in length, thickness and width are formed with holes, so as to output a desired magnetic field correction component.
The techniques of Patent Documents 1 and 2 serve to set the mounting position of the magnetic material shims, and the technique of Patent Document 3 serves to adjust the magnetic field correction component of the magnetic material shims. In general, in a state where the superconducting magnet is excited to generate the static magnetic field, an intense electromagnetic force acts on the magnetic material shims, and hence, the magnetic material shims cannot be safely arranged and adjusted. Accordingly, the adjustment of the position of the magnetic material shims according to Patent Document 1 or 2 and the adjustment of the volume of the magnetic material according to Patent Document 3 are made in a state where the superconducting magnet is once deexcited. Thereafter, the superconducting magnet needs to be excited again for confirming the magnetic field strength. However, a long time, for example, about one hour is expended on each of the excitation and deexcitation of the superconducting magnet, and a time period of two hours or more is required for deexciting the superconducting magnet once and exciting it again. As another problem, in deexciting and exciting the superconducting magnet, large currents must be caused to flow through current leads made of a normal conductor. Joule heat is generated within the superconducting magnet by the large currents, and liquid helium is vaporized and decreased by the Joule heat.